In primary cultures of embryonic mesencephalic neurons nitric oxide (NO) facilitates dopamine release in a dose-dependent manner. A battery of experiments excluded the involvement of increasec Ca2+ currents or membrane action potential, arachidonic acid release, filamentous actin disassembly, and pH changes. No increased the transfer of radiolabeled NAD onto a 39 kD and 24 kD protein from cytosol fraction of striatal synaptosomes, but a functional link to dopamine release could not be determined. Under anaerobic condition indicated that the NO radical acted directly on a target site instead of a reactive NO/oxygen intermediate. These results suggest that a heme- or metallo-protein may be the mediator.